DESCRIPTION (Verbatim From the Applicant's Abstract): The goal of our studies is to determine where and how genetic and epigenetic factors interact to result in sexual differentiation of the brain. Sex differences and hormone actions in brain development and organization may underlie selective vulnerabilities to major mental disorders. The proposed experiments will utilize time-lapse video microscopy in vitro to follow cells labeled with the fluorescent dye, DiI, assessing the extent and direction of their movements. Cell migration will be viewed in brain slice preparations taken form the preoptic area and hypothalamus (POA/AH) of developing mice during a critical period of sexual differentiation. The POA/AH is a site where hormones dramatically influence development. Cells will also be identified based on cell phenotype, i.e. the presence of specific proteins or mRNAs in vitro and in littermate embryos in vivo. Age dependent changes in the position of identified cells will be quantified to test the hypotheses using this independent index of cell migration. This application exploits the power of mouse genetics provided by characterized inbred strains and transgenic and gene-disrupted lines. The proposed studies focus on three basic questions:(1) Are genetic and/or sex differences in POA/AH organization contingent upon strain-dependent characteristics of cell migration? (2) Are sex differences in the migration of cells in the embryonic POA/AH regulated by gonadal steroids?; and (3) Do neurotransmitter actions contribute to the development of the POA/AH by influencing the characteristics of cell migration, or the phenotypic differentiation of cells? These studies will aid our understanding of the organization of specific subgroups of cells in the POA/AH, which may have specific physiological or behavioral functions. These studies will bring us closer to determining the pattern forming steps involved in the establishment of sexual dimorphisms and the actions of steroid hormones on the developing nervous system.